1. Field of the Invention:
The present invention relates to an improved papermaking technique and to the final product paper sheet material provided thereby.
2. Description of the Prior Art:
In conventional papermaking processes, raw paper pulp essentially consisting of cellulosic fibers is formulated into a dilute aqueous suspension which is introduced into the headbox of the papermaking machine from whence it is distributed over a filtering surface, e.g., Fourdrinier wire, on which the web of paper forms. The web is then drained and dried. The quality and properties of the resulting paper are determined, notably, by the operating conditions of the papermaking machine, the state of the raw pulp, the various additives that are added to the suspension prior to the formation of the web, as well as the materials that are deposited onto the paper web downstream of the filter surface.
Among such additives that are formulated into the fiber suspension introduced into the headbox, exemplary are mineral fillers, sizing agents, agents for enhancing the mechanical characteristics of the paper, coloring agents, and the like. The mineral fillers, such as CaCO.sub.3, TiO.sub.2, etc., render the sheet of paper opaque, thus facilitating writing and printing. Sizing agents, such as the alkylketene dimers, are added to impart liquid resistance properties to the sheet, and to permit the sized sheet of paper to be employed for writing and printing.
One of the qualities of paper, paperboard and other such products is the regularity of its characteristics. Among these is the so-called "look-thru" or "sheet formation" which is extremely important and is representative of the general quality of the paper. The look-thru effectively represents a greater or lesser degree of homogeneity of fiber distribution over the surface and thickness of the sheet. The look-thru quality will hence have considerable influence on the general quality of the print in the case, for example, of paper used for printing and writing, and on mechanical properties in the case, for example, of paper used for packaging. In order to improve the look-thru, more and more additives are being added to the suspension introduced into the headbox. But, in order to meet the increasing strictness of pollution standards requirements, papermaking plants are attempting to decrease their water consumption, which results in an increase in the pollutant loading associated with this type of industry (suspended matter, biological oxygen requirement, chemical oxygen requirement, salinity, etc.). Other problems are also encountered, such as the problem of pitches, rosins and the like, and, more generally, pollutants associated with the presence of so-called anionic trash. As the characteristics of the paper must, however, remain identical and the problems associated with preserving the environment have to be overcome, one of the solutions proposed in the prior art entails increasing retention of the above elements within the web during formation thereof on the filtering surface. This result is attained to a certain degree, by adding retention agents to the fiber suspension introduced into the headbox. Unfortunately, this technique is subject either to variations in its effectiveness, or results in a significant deterioration in the look-thru of the paper, depending on the products used.
Thus, serious need continues to exist in this art for an improved technique for producing paper and the like, in which, in addition to the mineral filler and the sizing agent, unique retention systems are admixed with the fiber suspension or composition introduced into the headbox.
EP-A-348,366 describes a ternary such system comprising cationic starch, a polyaluminum compound, and a silicic acid polymer. No anionic silica is indicated and, in addition, the polysilicic acid must have a given specific surface area (&gt;1,050 M.sup.2 /g).
WO 88/6,659 describes a ternary system comprising a cationic polymer, polyaluminum compound, and silica. The cationic polymer is preferably polyacrylamide and cationic starch is again not indicated.
EP-A-285,486 describes a retention system based on cationic starch and polyaluminum chloride. The mineral filler can, for example, be silica.
U.S. Pat. No. 4,643,801 describes a retention system based on cationic starch, a high molecular weight anionic polymer and silica, in parts by weight of starch/silica of 100/1 to 1/1 and of anionic polymer/silica from 20/1 to 1/10. An aluminum compound can also be added.
In "polyaluminum hydroxychloride application on neutral pH rosin sizing of paper" by B. H. Wortley and J. C. Steelhammer, a retention system is indicated comprising cationic starch, bentonite and an anionic polymer, but not polyaluminum chloride (PAC).